1. Field of the Invention
The present invention relates to current-perpendicular-to-the-plane (CPP) giant magnetoresistive elements in which a sensing current flows in the thickness direction (perpendicular to the planes) of the individual layers constituting the elements.
2. Description of the Related Art
Giant magnetoresistive (GMR) elements used for hard disk drives, magnetic sensors, etc., can be classified into two groups, i.e., current-in-the-plane (CIP) elements in which a sensing current flows parallel to the planes of the individual layers constituting the elements, and current-perpendicular-to-the-plane (CPP) elements in which a sensing current flows perpendicular to the planes of the individual layers constituting the elements.
CIP-GMR elements are commonly used products. However, in the CIP-GMR element, as the track width is narrowed, the output (the rate of change in resistance ΔR/R) is decreased. Therefore, if an attempt is made to further narrow the track width, various problems will occur. In contrast, in the CPP-GMR element, the output (the change in resistance per unit area ΔR·A) varies depending on the thickness of the element. At a constant current density, even if the track width is narrowed, the output does not change.
CPP-GMR elements in which the element output does not depend on the track width are thought to be more suitable for track narrowing compared to CIP-GMR elements and have been receiving attention recently. Conventional CPP-GMR elements are disclosed in Japanese Unexamined Patent Application Publication No. 2002-175611, etc.
For example, a so-called dual spin-valve CPP-GMR element typically includes a multilayer film in which a lower antiferromagnetic layer, a lower pinned magnetic layer, a lower nonmagnetic layer, a free magnetic layer, an upper nonmagnetic layer, an upper pinned magnetic layer, and an upper antiferromagnetic layer are disposed in that order from the bottom; first and second electrode layers disposed on the top and bottom of the multilayer film; hard bias layers disposed on both sides of the free magnetic layer; and insulating layers disposed above and below the hard bias layers. Each of the lower pinned magnetic layer and the upper pinned magnetic layer often has a laminated ferrimagnetic structure which includes a first pinned magnetic sublayer in contact with the lower antiferromagnetic layer or the upper antiferromagnetic layer, a nonmagnetic intermediate sublayer disposed on the first pinned magnetic sublayer, and a second pinned magnetic sublayer disposed on the first pinned magnetic sublayer with the nonmagnetic intermediate sublayer therebetween. Each of the upper antiferromagnetic layer and the lower antiferromagnetic layer is, for example, composed of PtMn; each of the upper nonmagnetic layer and the lower nonmagnetic layer is, for example, composed of Cu; and each of the upper nonmagnetic intermediate sublayer and the lower nonmagnetic intermediate sublayer is, for example, composed of Ru. Each of the upper first pinned magnetic sublayer, the lower first pinned magnetic sublayer, the upper second pinned magnetic sublayer, the lower second pinned magnetic sublayer, and the free magnetic layer is, for example, composed of CoFe or NiFe.
In the conventional CPP-GMR element having the structure described above, under the present situation, it is not possible to achieve an element output that is sufficient for practical use. The element output is proportional to the intensity of the current flowing through the element and the change in resistance per unit area ΔR·A. In order to increase the element output, the sensing current must be increased or the change in resistance per unit area ΔR·A must be increased. However, if the sensing current is increased, the CPP-GMR element generates heat, resulting in a decrease in the output. Therefore, it is not possible to increase the sensing current above a certain value. Accordingly, how the change in resistance per unit area ΔR·A is improved is a subject to be studied.
Recently, on the assumption that if the thickness of a CPP-GMR element (in particular, the thicknesses of the layers contributing to the magnetoresistance effect) is increased, the change in resistance per unit area ΔR·A will be increased, the free magnetic layer and the second pinned magnetic sublayer (the lower second pinned magnetic sublayer and the upper second pinned magnetic sublayer in the dual type) are formed such that they have large thicknesses. Specifically, each of the free magnetic layer and the second pinned magnetic sublayer is formed so as to have a three-layered structure, for example, composed of CoFe/NiFe/CoFe, in which the thickness of the NiFe portion is set at about 40 to 100 Å, the NiFe portion having a larger spin-dependent bulk scattering coefficient β than that of the CoFe portion.
However, if the thicknesses of the free magnetic layer and the second pinned magnetic sublayer are increased, it becomes difficult to place the CPP-GMR element between the upper shielding layer and the lower shielding layer of a magnetic head. Since the distance between the upper shielding layer and the lower shielding layer defines the track recording density, it is not possible to greatly change the distance.
If the thickness of the free magnetic layer is increased, the magnetic thickness (magnetic moment per unit area; saturation magnetization Ms×thickness t) is also increased. Consequently, the magnetization of the free magnetic layer is not easily rotated in response to a very small magnetic field from outside (a recording medium), resulting in a decrease in output sensitivity.
On the other hand, in the pinned magnetic layer (the upper pinned magnetic layer and the lower pinned magnetic layer in the dual type), if the thickness of the second pinned magnetic sublayer is increased, the magnetic thickness of the second pinned magnetic sublayer is increased, and the exchange coupling magnetic field which maintains the RKKY antiparallel state of the first pinned magnetic sublayer and the second pinned magnetic sublayer being interposed by the nonmagnetic intermediate sublayer is decreased. If the exchange coupling magnetic field is decreased, the magnetization of the second pinned magnetic sublayer is greatly inclined due to the longitudinal bias magnetic field of the hard bias layer, resulting in an increase in output asymmetry. There is also a possibility that the magnetization of the second pinned magnetic sublayer is reversed due to electrostatic discharge (ESD), resulting in a degradation in reliability.
As described above, if the thicknesses of the free magnetic layer and the pinned magnetic layer (in particular, the second pinned magnetic sublayer) are increased, although the change in resistance per unit area ΔR·A is increased, various disadvantageous effects are caused.